The present invention generally relates to position coordinate input devices and more particularly, to a stress detection type position coordinate input device which determines, by detecting component forces of an applied external force with the use of a plurality of stress detectors, coordinates of a position indicated by a finger, a pointing rod, etc. and then, inputs the coordinates to a computer and the like.
Conventionally, as position coordinate input devices for use with computers, etc., there have been employed so-called digitizers which are expensive but are capable of accurately inputting coordinates of graphic forms. However, as computers have come into general use recently, there has been a demand for inexpensive position coordinate input devices. Especially, there is a strong demand for penetrating type position coordinate input devices by which data can be inputted so as to penetrate into their display units by depressing, with a finger, etc., graphic forms displayed on the display units. This is because such a method is generally adopted of late that, in order to enable even persons unfamiliar with operation of computers to operate them with ease, several selective branches prepared beforehand are displayed on the display units such that processing is automatically performed by depressing one of the selective branches with a finger, etc. Light pens have been typically used as such penetrating type position coordinate input devices. However, there have been to data proposed various devices adopting more convenient methods in which data can be inputted by a finger, a ball-point pen, etc. without using the light pens. As typical ones of such methods, there can be recited a switch matrix method employing a transparent and electrically conductive sheet, a voltage/electric current divisional method, a method employing laser beams, a method in which a time period required for transmission of surface waves is measured, etc.
However, these prior art devices have such inconveniences that the devices are complicated in structure and expensive, that it is hard to view data displayed on the display units, that it is difficult to perform maintenance operations therefor, etc., thus preventing the prior art devices from coming into popular use.
In place of the above described prior art devices, there has been further proposed a stress detection type position coordinate input device in which, by detecting component forces of a force applied to a point on an input plate, coordinates of the point of application of the force are detected. The stress detection type position coordinate input device can employ an input plate made of uniform and entirely transparent materials such as a glass plate, an acrylic plate, etc. and is simple in structure, thus lowering its production cost. A device based upon such a principle is disclosed, for example, in Japanese Patent Publication No. 34247/1974 (Tokkosho 49-34247) and U.S. Pat. No. 4,389,711 in which, by detecting component forces of a force at three points on the input plate, coordinates of the point of application of the force are detected as shown in FIG. 1.
In FIG. 1, when a force F is applied to a point 2 on a rectangular input plate 1 having a longitudinal length H and a sidewise length 2L, component forces f1, f2 and f3 of the force F are measured at points 3a, 3b and 3c, respectively. It is to be noted that the point 3c is located at an uppermost central point of the input plate 1. Thus, when Cartesian coordinates X and Y are established by setting the point 3a at the origin as shown, the points 3a, 3b and 3c assume coordinates (0,0), (2L,0) and (L,H), respectively. Consequently, coordinates (x,y) of the point 2 of application of the force F are given by the following equations. ##EQU1##
Accordingly, it has become apparent that the coordinates (x,y) of the point 2 of application of the force F can be determined by detecting the component forces at three points. However, this known device has a disadvantage that such factors as weight of the input plate 1, variations of detection outputs of the component forces in response to temperature changes, errors due to strain of the input plate 1 itself, etc. are required to be taken into account in regard to determination of the point 2 of application of the force F. Furthermore, the known device has been disadvantageous in that, since strain of the input plate 1 increases upon application of the force F to such an end portion of the input plate 1 as a point 4 (FIG. 1) in the case where the input plate 1 has a rectangular shape, the input plate 1 is required to be increased in strength as the input plate 1 becomes large in size.